1. Field of the Invention
The present invention relates a voltage detecting circuit, and particularly relates to a voltage detecting circuit that measures a voltage of each battery cell of a plurality of battery cells connected in series.
2. Description of Related Art
In recent years, lithium ion batteries have been used for the purpose of being mounted on a vehicle such as a hybrid car. However, since the lithium ion batteries have a limitation of a voltage that can be outputted by one cell, a battery pack of the battery cells connected in a multi-stage is used as a battery for use in a vehicle that needs high voltage, or for other similar usage. Here, when an overcharge occurs in such a lithium ion battery, safety and life thereof may deteriorate. For that reason, the battery pack using the lithium ion battery needs to avoid risks caused by overcharge and the like by monitoring a voltage value outputted from each battery cell.
To this end, there have been proposed a variety of voltage detecting circuits that detect the voltages of battery cells included in a battery pack. As for these voltage detecting circuits, one voltage detecting circuit is designed to detect voltages of multiple battery cells in order to reduce the number of components of a power source module including the battery pack and the voltage detecting circuit. When one voltage detecting circuit measures multiple battery cells, as described above, the voltage detecting circuit is provided with, as a ground voltage, a voltage of a negative electrode terminal of a battery cell disposed on the lowest potential side of the measured battery cells, and is provided with, as a supply voltage, a voltage of a positive electrode terminal of a battery cell disposed on the highest potential side of the measured battery cells. For this reason, when the voltages of battery cells are to be measured separately, a circuit that forms a measuring part needs to be formed of a high breakdown voltage element having a breakdown voltage higher than a voltage outputted by one battery cell. Since the high breakdown voltage element generally has a large size, there has been a problem that a voltage detecting circuit formed by using many high breakdown voltage elements has a large chip size.
In order to form the measuring part by use of a low breakdown voltage element, a “flying capacitor” method has been proposed as a measurement method for a voltage detecting circuit. In the flying capacitor method, a voltage outputted from one battery cell is sampled by a sampling capacitor, and the sampled voltage value is measured by the measuring part such as an analog-to-digital converter (A/D). Since this limits a voltage value inputted into the measuring part to a voltage range outputted from one battery cell, the measuring part can be formed of the low breakdown voltage element.
On the other hand, in order to efficiently use the lithium ion battery, it is necessary to measure a voltage value outputted by a battery cell with high accuracy. In the flying capacitor method, a large number of switches are used, and these switches have parasitic capacitance. For this reason, measurement by the flying capacitor method has a problem that the voltage value sampled by the sampling capacitor fluctuates due to this parasitic capacitance. Thus, the accuracy of measurement deteriorates. JP-A-2001-201522 has disclosed a technique that improves accuracy of measurement by the voltage detecting circuit in the flying capacitor method.
A circuit diagram of a cell voltage detecting circuit 100 disclosed in JP-A-2001-201522 is shown in FIG. 6. The cell voltage detecting circuit 100 separately measures voltage values of respective cells #(1) to #(N), which form a cell group 101, by measurement using the flying capacitor method. As shown in FIG. 6, the cell voltage detecting circuit 100 has cell selection switches 102 and 103, a sampling switch 104, a sampling capacitor 105, a transfer switch 106, and an A/D 107. Moreover, parasitic capacitance Cs is added to each switch shown in FIG. 6.
The cell selection switch 102 has switches SL(1) to SL(N) connected to negative electrode terminals of the cells #(1) to #(N), respectively. The cell selection switch 103 has switches SU(1) to SU(N) connected to positive electrode terminals of the cells #(1) to #(N), respectively. Here, the cell selection switch selects a single cell or multiple cells within the cell group 101.
The sampling switch 104 has a switch Spl-U provided corresponding to the positive electrode terminals of the cells (or a high potential side terminal of the sampling capacitor 105) and a switch Spl-L provided corresponding to the negative electrode terminals of the cells (or a low potential side terminal of the sampling capacitor 105). Here, the sampling switch 104 provides terminals on both sides of the sampling capacitor 105 with a potential difference between the negative electrode terminal and the positive electrode terminal of the cell selected by the cell selection switch. The sampling capacitor 105 is charged with a voltage sampled by the sampling switch 104.
The transfer switch 106 has a switch Trns-U provided corresponding to the positive electrode terminals of the cells (or the high potential side terminal of the sampling capacitor 105) and a switch Trns-L provided corresponding to the negative electrode terminals of the cells (or the low potential side terminal of the sampling capacitor 105). Here, after the sampling switch 104 changes to OFF, the transfer switch 106 transfers the voltages in both ends of the sampling capacitor 105 to output terminals of the transfer switch 106. The A/D 107 converts the potential difference transferred to the output terminals of the transfer switch 106 to a digital value, and outputs the digital value.
In other words, in the cell voltage detecting circuit 100, after the sampling capacitor 105 samples the voltage value of the cell, the sampling switch 104 is turned off to prevent the parasitic capacitance Cs of the cell selection switch from influencing subsequent measurement. This reduces a measurement error caused by the parasitic capacitance Cs of the cell selection switch in the cell voltage detecting circuit 100.